How did Fury climb the ranks in 15 years?

He thought it was a kitty, but it was something else entirely. 

An animal does not have to appear threatening to pose a threat, there are a lot of animals on this Earth that look quite harmless but if provoked can attack and end your life - like a lion, for instance. A lion does not look scary if its minding its own business and has its mouth shut, but if angered, running, and has its mouth wide open then yes it is very scary and nobody wants to be the target. 

Or, look at the Tasmanian devil, sure they look cute enough to where you might want to pet them and maybe play with them but it is highly recommended to avoid being near one in the wild as they are serous meat eaters and their teeth can chomp right into some of the most hardest of known textures.